The present invention is related to apparatus and methods to drill and complete oil and gas wells. More particularly, the present invention is related to using a casing string to both drill a wellbore and complete the wellbore.
Oil and gas wells and other types of wells such as geothermal wells are typically drilled from the well surface to a desired downhole location using a rotary drilling rig, drill pipe, and drill bits. Various types of bottom hole assemblies may also be used as required by associated geological formations. After a wellbore has been drilled to a desired depth or downhole location, one or more casing strings may then be installed extending from the well surface to the desired downhole location. A cement sheath or cement bond is typically formed between the outside diameter of each casing string and adjacent inside diameters of the wellbore. Many wells have two or more casing strings with different inside diameters installed in a telescoping manner from the well surface to a desired downhole location. For example, a first casing string may be installed from the well surface to a depth of 500 to 1,000 feet. The first casing string may have an outside diameter of nine and five/eighths inches (9⅝xe2x80x3). A second casing string may be installed extending from proximate the downhole end of the first casing string to a desired depth of 9,000 or 10,000 feet. The second casing string may have an outside diameter of approximately seven inches (7xe2x80x3).
Wellbores for producing oil, gas or other formation fluids from selected subsurface formations, are typically drilled in stages. For example, a wellbore may be first drilled with a drill string and a first drill bit having particular diameter. At a desired depth for a first portion of the wellbore, the drill string and drill bit are removed from the wellbore. Tubular members of smaller diameter, often referred to as casing or a casing string, may then be placed in the first portion of the wellbore. An annulus formed between the inside diameter of the wellbore and the outside diameter of the casing string is generally filled with cement. The cement provides support for the casing and isolates downhole formations or subterranean strata from each other. Often, the next step in drilling the wellbore is to pass the drill string with a second, smaller diameter drill bit through the first casing string and drill another portion of the wellbore to a selected depth beyond the depth of the first casing string. This sequence of drilling wellbores and installing casing strings may be repeated as many times as necessary, with smaller and smaller components until the ultimate desired depth or downhole location of the wellbore has been achieved.
The cost of drilling and completing wellbores, particularly in offshore regions, is extremely expensive. Typical operating costs for an offshore drilling rig may easily exceed $100,000 per day. Thus, there are substantial benefits to minimizing the length of time required to drill a wellbore. At extended distances from the well surface (15,000 feet or greater) round trip time required to pull a drill bit and replace it with another drill bit may be several hours. The associated trip time is often non-productive and wasteful.
For very deep wells or very long wells (20,000 feet or greater), there may be three or four changes in casing diameter from the well surface to the total depth or desired downhole location of the wellbore. Each change in diameter of the casing string results in a corresponding decrease in the diameter of production tubing used to produce formation fluids from selected downhole formations. The change in casing diameter associated with typical deep wells and/or long wells results in significantly increased drilling and completion costs.
During the past several years an increasing number of oil and gas wells have been drilled using standard oilfield casing rather than drill pipe. Standard oilfield casing generally has less wall thickness when compared with standard drill pipe. Drill bits and other downhole tools used to drill wells have been modified to allow installation and removal from downhole locations through a casing string using wireline techniques. For example, drill bits may be inserted through the longitudinal bore of a casing string and releasably latched adjacent to and extending from the end of to the last joint of a casing string using wireline techniques. Such drill bits may also be removed from the end of the casing string using similar wireline techniques when the wellbore has been drilled to a desired downhole location. The casing string may then be cemented within the wellbore to form a portion of the well completion system.
Drilling a wellbore with a casing string eliminates or substantially reduces the number of times which pipe must be removed and inserted (tripped) from the wellbore. Drilling a well using a casing string generally reduces drilling costs and completion costs as compared with drilling a well with conventional drill pipe and using conventional completion techniques to install a casing string in the resulting wellbore.
In accordance with teachings of the present invention, threaded connections are provided for use on casing and other tubular goods used for drilling and completing wellbores. One aspect of the present invention includes providing tubular goods with threaded and coupled connections which may be used to releasably engage the tubular goods with each other to allow drilling a wellbore and completion the wellbore.
Technical benefits of the present invention include providing a casing string with threaded and coupled connections that enhance use of the casing string to drill a wellbore and complete the wellbore without removing or tripping the casing string. The threaded and coupled connection may be substantially stronger and more rugged than conventional threaded connections used on oilfield casing. A threaded connection formed in accordance with teachings of the present invention provides mechanical strength required to drill a wellbore and provides fluid, pressure tight seals between the interior and the exterior of the casing string. Since the wall thickness of a casing string is generally less than the wall thickness of conventional drill pipe, threaded and coupled connections formed in accordance with teachings of the present invention provided desired mechanical strength in threaded portions where the wall thickness is often reduced as compared with other portions of the casing string.
Further technical benefits of the present invention include providing one or more mechanical surfaces on the exterior of a casing string to accommodate applying hardfacing to selected portions of the casing string. Hardfacing selected portions of a casing string may be desired when the casing string is used to drill a wellbore.